Compressed gasses such as hydrogen, methane, and natural gas (CNG) are suitable fuels for internal combustion engines. In some localities, gaseous fuels may be less expensive than gasoline or diesel fuel in terms of their cost per energy equivalent. Further, in contrast to liquid fuels, gaseous fuel may be more accurately controlled at or near stoichiometry during an engine start, as less excess fueling is used to achieve desired combustion performance. Moreover, some gaseous fuels may have higher octane ratings than liquid fuels.
A challenge for gaseous fuel systems in motor vehicles is the low and temperature-dependent volumetric energy density of compressed-gas fuels. One consequence is that the mechanical work needed to fill a compressed-gas fuel tank may be a significant fraction of the total internal energy stored in the fuel tank. Further, as the density of a gas at constant pressure decreases with increasing temperature, the very act of filling the fuel tank may, under some conditions, increase the temperature of the gas such that a fuel tank filled to a constant pressure contains less fuel mass than would be present if the fuel tank was filled to the same pressure at ambient temperature.
Various attempts to address these issues have appeared. For example, U.S. Patent Application Publication 2007/0000563 provides a system for increasing the overall efficiency of a high-pressure gas-fueled vehicle and refilling station infrastructure. In the disclosed system, the evolved heat from high-pressure refueling is absorbed by a melting/solidifying medium inside the fuel tank, and may be dispersed via an external radiator. In this manner, a denser charge of fuel may be admitted to the fuel tank.
However, the approach cited above requires extensive hardware and special materials dedicated exclusively to temperature management. The inventors herein have recognized this limitation and have provided a more elegant approach that may be implemented in a motor-vehicle fuel system having multiple compressed-gas fuel tanks. Further, the system may be integrated with a method for minimizing the mechanical work needed to refill a fuel system having multiple fuel tanks.
Therefore, in one embodiment, a method for distributing fuel in a fuel system of a motor vehicle is provided. The method may be applied in a fuel system having a first fuel tank, where fuel is confined at a first pressure, and a second fuel tank, where fuel is confined at a second pressure greater than the first pressure. The method comprises releasing fuel already resident in the second fuel tank to the first fuel tank, and admitting fuel to the first and second fuel tanks simultaneously. Other embodiments provide other, more particular methods for distributing fuel in a fuel system of a motor vehicle. In this manner, a more simply configured fuel system may be operated to accommodate a denser charge of fuel than might otherwise be possible.
It will be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description, which follows. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined by the claims that follow the detailed description. Further, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.